1. Field of the Invention
The present invention relates to a signal transmission circuit and, more particularly, to a circuit controlling an impedance for transmission of a signal on a transmission line.
2. Description of the Related Art
In connection with a signal transmission circuit in a bidirectional transmission line, deterioration of a transmission characteristic can be prevented if one of the transmitting or receiving devices enters a transmitting state, the other transmitting receiving device is set to a receiving state. Reference can be made, for example, to JP-A-07-107020.
To be more specific, in a constitution in which a pair of transmitting/receiving devices is connected to both terminals of a transmission line, a switch and a terminating resistor are interposed between each of the terminals of the transmission line and a terminating voltage source. If each of the transmitting/receiving devices enters a transmitting state, the switch is turned off. If each of the transmitting/receiving devices enters a receiving state, the switch is turned on. Thus, far-end termination can be realized, and a waveform distortion can be reduced.
The foregoing conventional signal transmission circuit presupposes that the number of transmitting/receiving devices that enter the transmitting state for one-to-one communication is always one. After the transmitting/receiving device that enters the transmitting state is identified, a transmitting/receiving device that enters the receiving state is terminated. Therefore, as for a large-scale network or a multi-master network, if transmitting/receiving devices that enter the transmitting or receiving state are uncertain, the conventional signal transmission circuit cannot be adopted. Assuming that an impedance mismatch point exists in a transmission line and a reflected wave is generated, the waveform distortion is increased because of the mismatch in the impedances at the transmitting end. Further, if multiple transmitting/receiving devices enter the receiving state, multiple terminating resistors are validated. A synthetic value of impedances decreases, and a signal voltage of a high level H decreases.